Trying to do a discovery on whether lapping an upper receiver increases accuracy.

If not, what other things can be done to accurize an AR?

I have read matching a bolt to a barrel, how is that accomplished?

Buy a really good scope and barrel.

The answer is a great big maybe.

It can’t hurt to ensure the face of the upper is square to the bore of the upper.  But if you’re using a bargain bin barrel, why bother?

If you’re going to spend large on the barrel, then go ahead and true the front of your upper.  Do so VERY carefully, removing only the tiniest amount of coating as you go.

The Frankford Arsenal tool says it’s made to be used with a power drill - I think that’s a dumb idea.  If you advertise something as “hook it up to your power drill”, a bunch of Bubbas are going to spin that thing at 10,000 RPM and erase too much metal...  Whatever tool you use, turn it by hand.  It’s slower, but hand turning gives you so much more control that it should be well worth it.

Quality barrel, scope, and trigger. Will give you the best accuracy. Followed by free floating which everyone does now.

I lap the front of the upper receiver on all my ARs, but I have never seen an increase in accuracy, at least in the first dozen which I did not lap before building.  IMO all it does is enable your sights, iron and optical, to be zeroed closer to their mid range of adjustment.  That’s enough for me.
I have found that most of your accuracy is in the barrel, trigger, and shooter.

How do you do barrel and bolt matching?

Check headspace using headspace gauges. If you buy the barrel and the bolt from the same company, they may offer to check for your. Rainier Arms does. Many others do too.

This is how I understand it too.  Also, when people are complaining about the barrel not being centered in their free float it is usually because of this.

If you have one good scope that you want to move from rifle to rifle, rather than buying a bunch of high end scopes, then it's essential.  Mount the lapping tool upright in a bench vise and then lower the receiver onto it and lap the face by hand, check your progress often and stop when you first get clean metal all the way around.  Often you're just reducing the irregularities from the anodizing process.

My lapping tool has paid for itself many times over.   It gets my irons closer to mechanical zero and barrels in the middle of free float handguards.
My most recent Aero upper purchase was the worst I have seen so far with the face being way out of square on one side.   It took me forever to get the long side cut down and square.
The lapping tool has been the most helpful to me personally for getting barrel nuts to line up, which I seem to have bad luck with.
If I buy a complete upper and it zero's and shoots fine I won't mess with it.
Of the uppers I have built, none of them have been square, even from quality manufacturers, so it's nice to know you've done everything you can to make something that much better.
I usually have to slightly lap the bar into the receiver raceway and the advantage to that is it will take down high spots due to the manufacturing process.   Usually around the ejection port there is some deformation.   If there are any inconsistencies where the bolt carrier bosses ride it will even them out and make cycling feel very smooth.
I lap by hand and using a drill just causes me problems, usually because the mandrel fit is so tight in the receiver that the friction created with the drill heats it up and it gets hard to turn/remove.
Sorry for the e-book, I just really like my lapping tool and think it's very useful to own.